The invention relates to a method of making an amorphous silicon dioxide free of metal ions, which in aqueous suspension is suitable for the mechanical polishing of semiconductor surfaces.
In the manufacture of semiconductor devices, thin monocrystalline silicon wafers are made by cutting a crystal of monocrystalline silicon. For further processing these wafers are fixed with was on a polishing block. The exposed surface of each wafer is then polished in order to remove the surface irregularities which are caused by the cutting of the crystal of monocrystalline silicon. Generally, the silicon wafers are processed in successive steps with grinding or polishing agents of different particle size. These process steps comprise the lapping and/or the so-called Blanchard grinding by which rough irregularities are removed from the surface, and finally one or several mechanical or chemical precision polishing processes which produce an extremely smooth and defect-free surface which subsequently is subjected to the known processing methods for making semiconductor devices.
As the polished surface of a silicon wafer is later supplied with a mask and etched, it is necessary for it to show a minimum of scratches and irregularities. Many attempts have been made to make high quality surfaces with different grinding and polishing agents. Polishing agents are known for the mechanical and/or chemical precision polishing process which contain aluminum oxide in alpha or gamma form, ceric oxide, copper-II-ions, diamond dust, silica sols or gels, or vitreous silicon dioxide and zirconium dioxide or zirconium silicate. It is furthermore known to fix the pH value of the polishing agents with alkali such as sodium hydroxide or with amines in a pH range between 9 and 12, or to add to the polishing agent an oxidizing compound, such as alkali peroxides or alkaline earth peroxides or organic oxidizing compounds in order to obtain a maximum abrasion per time unit. For prior art, reference is made to the article by E. Mendel, "Polishing of Silicon", is SCP and Solid State Technology, August 1967, pp. 27-39. The prior art is furthermore described in the following patents: U.S. Pat. Nos. 2,744,001, 3,071,455, GEP No. 1,621,473, U.S. Pat. No. 3,328,141, and GE-OS No. 1,644,725, GE-OS No. 2,305,188, and GEP No. 1,271,288.
For polishing silicon wafers, polishing agents containing a silicon dioxide powder are highly desirable because silicon dioxide shows a Mohs hardness of 7 which is very close to the hardness value of silicon and therefore shows a relatively high material abrasion rate. During the polishing process it is, therefore, not embedded in the surface of the polished silicon wafers. However, a disadvantage of the silicon dioxide-containing polishing agents is their sodium ion content which is due to the presence of Na.sub.y (SiO.sub.x) compounds in the polishing suspension. Low quantities of the sodium ions adhere to the polished silicon surface and diffuse into the substrate during the subsequent high temperature process. Due to the high integration density of integrated circuits, however, even these low quantities of metal ions cause high leakage currents and thus the failure of the highly integrated circuits.